1. Field of the Invention
The present invention relates generally to a zoom lens used with an image pickup device, and more particularly to a zoom lens using a diffractive optical element, which is suitable for use with video cameras or digital still cameras, as well as an image pickup device using such a zoom lens.
2. Discussion of the Related Art
For recently developed zoom lenses for image pickup devices using image pickup elements such as CCDs, there are proposed a number of zoom lens systems, each using four lens groups, i.e., comprising, in order from its object side, a first positive lens group, a second negative lens group, a third positive or negative lens group and a fourth positive or negative lens group. For such a zoom lens system it is preferable that aberrations in each group are reduced as much as possible. To this end it is required that each group be constructed of a plurality of positive and negative lenses. If a diffractive optical element (hereinafter DOE for short) is used in such a zoom lens system, however, it is then possible to make satisfactory correction for chromatic aberrations with no increase in the number of lenses used, as already proposed in the art. For instance, JP-A 9-211329 discloses a zoom lens system in which a DOE is used in the first or second lens group. However, if the diffraction efficiency of the DOE with respect to design order-of-diffraction light, then the intensity of light other than the design order-of-diffraction light (hereinafter called the unnecessary light) increases, resulting in a failure in obtaining sufficient image quality. Especially to enable the DOE to be used in a wide wavelength region (.lambda.=about 400 nm to about 700 nm) where phototaking lenses are used, the DOE should have high diffraction efficiency. If the DOE is of sawtooth shape in section, it is then possible to improve its diffraction efficiency. For instance, 100% diffraction efficiency can be theoretically obtained at one wavelength and one angle of view. However, as the angle of incidence of a ray bundle on a DOE diffractive surface becomes large, the diffraction efficiency drops drastically. For further information about this, see articles "Scalar theory of transmission relief gratings", Optics Communications, Vol. 80, No. 5, 6/307-311 (1991), "Blazed holographic gratings for polychromatic and miltidirectional incident light", J. Opt. Soc. Am., Vol. 9, No. 7/1196-1199 (1992), etc. Thus, the zoom lens system with the DOE used in the first lens group, disclosed in JP-A 9-211329, cannot immediately be used because the angle of incidence of the ray bundle on the diffractive surface is very large. When the DOE is used in the first lens group of a zoom lens system, the angle of incidence of a ray bundle on a DOE diffractive surface must be made as small as possible because that angle of incidence varies largely from a wide-angle side to a telephoto side of the zoom lens system.
Unless the sufficient number of DOE gratings is ensured with respect to a ray bundle incident on the diffractive surface, then no sufficient image quality is obtained because the intensity distribution of design order-of-diffraction light spreads wide. For further information about this, see an article "Rigorous electromagnetic analysis of diffraction by finite-number-of-periods gratings", J. Opt. Soc. Am. A/Vol. 14, No. 4/907-917 (1997). Thus, the use of the DOE in the first or second lens group of the zoom lens system causes deterioration of image quality, because it is impossible to ensure the sufficient number of gratings with respect to a ray bundle in the vicinity of the optical axis on the wide-angle side in particular.
U.S. Pat. No. 5,268,790 comes up with a zoom lens system wherein DOEs are used in the second and third lens groups. However, the sufficient number of gratings cannot be obtained with respect to a ray bundle in the vicinity of the optical axis, because the powers of DOE diffractive surfaces are very weak and the number of gratings is very limited.